Astronomy & Astrophysics manuscript no. V4˙BSGhosts c ESO 2018 November 5, 2018 A metallicity study of 1987A-like supernova host galaxies? F. Taddia1, J. Sollerman1, A. Razza1, E. Gafton1, A. Pastorello2, C. Fransson1, M. D. Stritzinger3, G. Leloudas4;5, and M. Ergon1 1 The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden (e-mail: [email protected]) 2 INAF-Osservatorio Astronomico di Padova, Vicolo dellOsservatorio 5, 35122 Padova, Italy 3 Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark 4 The Oskar Klein Centre, Department of Physics, Stockholm University, AlbaNova, 10691 Stockholm, Sweden 5 Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark Received / Accepted Abstract Context. The origin of the blue supergiant (BSG) progenitor of Supernova (SN) 1987A has long been debated, along with the role that its sub-solar metallicity played. We now have a sample of SN 1987A-like events that arise from the rare core collapse (CC) of massive (∼20 M ) and compact (. 100 R ) BSGs. Aims. The metallicity of the explosion sites of the known BSG SNe is investigated, as well as the association of BSG SNe to star- forming regions. Methods. Both indirect and direct metallicity measurements of 13 BSG SN host galaxies are presented, and compared to those of other CC SN types. Indirect measurements are based on the known luminosity-metallicity relation and on published metallicity gradients of spiral galaxies. In order to provide direct metallicity measurements based on strong line diagnostics, we obtained spectra of each BSG SN host galaxy both at the exact SN explosion sites and at the positions of other H II regions. We also observed these hosts with narrow Hα and broad R−band filters in order to produce continuum-subtracted Hα images. This allows us to measure the degree of association between BSG SNe and star-forming regions, and to compare it to that of other SN types. Results. BSG SNe are found to explode either in low-luminosity galaxies or at large distances from the nuclei of luminous hosts. Therefore, their indirectly measured metallicities are typically lower than those of SNe IIP and Ibc. This result is confirmed by the direct metallicity estimates, which show slightly sub-solar oxygen abundances (12+log(O/H)∼8.3-8.4 dex) for the local environments of BSG SNe, similar to that of the Large Magellanic Cloud (LMC), where SN 1987A exploded. However, we also note that two objects of our sample (SNe 1998A and 2004em) were found at near solar metallicity. SNe IIb have a metallicity distribution similar to that of our BSG SNe. Finally, we find that the degree of association to star-forming regions is similar among BSG SNe, SNe IIP and IIn. Conclusions. Our results suggest that LMC metal abundances play a role in the formation of some 1987A-like SNe. This would naturally fit in a single star scenario for the progenitors. However, the existence of two events at nearly solar metallicity suggests that also other channels, e.g. binarity, contribute to produce BSG SNe. Key words. supernovae: general – supernovae: individual: SN 1909A, SN 1982F, SN 1987A, SN 1998A, SN 1998bt, SN 2000cb, OGLE-2003-NOOS-005, SN 2004ek, SN 2004em, SN 2005ci, SN 2005dp, SN 2006V, SN 2006au, SN 2009E – stars: evolution – galaxies: abundances 1. Introduction Many attempts have been made to explain the nature of the BSG progenitor of SN 1987A (see Podsiadlowski 1992 for a re- Supernova (SN) 1987A, the most studied SN ever, was also the view), since standard stellar models did not predict the explosion first object whose progenitor (Sanduleak −69◦202) could be di- of massive stars in the BSG phase (e.g. Woosley et al. 2002). rectly identified (Gilmozzi et al. 1987). Surprisingly, the mas- As a matter of fact, a single non-rotating ∼20 M star born in sive (MZAMS ∼ 16 − 22 M , Arnett et al. 1989) progenitor star a solar-metallicity environment is expected to be a RSG at the was not a red supergiant (RSG) with a radius of a few hundred time of collapse (Eldridge 2005). Therefore, special conditions R as envisioned by contemporary theory, but rather a compact are required for a massive star to be a BSG at the endpoint of its arXiv:1308.5545v1 [astro-ph.CO] 26 Aug 2013 (R . 50 R ) blue supergiant (BSG). The compactness of its pro- life. The possible scenarios include those that involve a single genitor explained both its slowly rising light curve (≈84 days) progenitor star and those that consider Sanduleak −69◦202 as and its low luminosity in the first months (e.g. Arnett et al. 1989). part of a binary system. We note that for the specific case of the Most of the kinetic energy was spent to adiabatically expand the progenitor of SN 1987A, any proposed scenario not only has to star while the light curve of SN 1987A was mainly powered by 56 56 explain why it was a BSG, but must also fit other constraints such the radioactive decay of Ni and Co. as the chemical anomalies (Saio et al. 1988; Fransson et al. 1989) and the peculiar CSM of SN 1987A (Morris & Podsiadlowski ? Based on observations performed at the Nordic Optical Telescope 2009). We now briefly review the possible mechanisms that lead (Proposal number 44-011 and 45-004, PI: F. Taddia; and 47-701), La to the explosion of a BSG according to theoretical models. Palma, Spain; the Very Large Telescope (Program 090.D-0092, PI: F. Taddia), Paranal, Chile. Some data were also obtained from the ESO In the single-star framework, a first natural explanation for Science Archive Facility and from the Isaac Newton Telescope archive. the BSG nature of SN 1987A’s progenitor was found in the sub- 1 F. Taddia et al.: A metallicity study of 1987A-like supernova host galaxies solar metallicity (12+log(O/H) = 8.37 dex, Russell & Dopita The aim of this paper is to provide an observational test of the 1990) measured in the Large Magellanic Cloud (LMC), which role of metallicity for BSG SNe, and we therefore perform a de- hosted SN 1987A (Langer 1991). A low metallicity can allow a tailed metallicity analysis of the galaxies that hosted a BSG SN. blue solution in the HR diagram for the endpoint of a massive In Sect. 2 details are given on our photometric and spectroscopic star (see e.g., Brunish & Truran 1982; Arnett 1987; Hillebrandt observations; Sect. 3 includes the results on the metallicity of et al. 1987; Woosley 1988). Another single star scenario that can the BSG SN explosion sites that were obtained indirectly from lead to the explosion of a BSG is that invoking a rotating star, the galaxy luminosity and the SN distance from its host center; where rotational mixing plays an important role in pushing the in Sect. 4 we present a metallicity study of the observed BSG stellar evolution towards the blue part of the HR diagram (see hosts using long slit spectroscopy. This is based on strong line di- e.g. Weiss et al. 1988; Hirschi et al. 2004; Ekstrom¨ et al. 2012). agnostics, including metallicity estimates at the exact explosion In the binary scenario, both accretion and merger models sites and a metallicity gradient for each host that was observed. may lead to the explosion of BSGs (see Woosley et al. 2002 Section 5 describes the measurements and the results concern- for references). We note that the binary solution was considered ing the association of BSG SNe to star-forming regions in their for the specific case of SN 1987A because it would more easily hosts. A discussion based on these results is given in Sect. 6 and explain the “hour-glass” shape of the CSM observed in its rem- our conclusions follow in Sect. 7. nant (Morris & Podsiadlowski 2009). As noted by Woosley et al. (2002), if massive stars end their lives as BSGs because they merge with a companion, we would expect to find BSG SNe also 2. Observations and data reduction at solar metallicity. A log of all the observations that were performed and collected From the observational point of view, testing the role of for this study is presented in Table 2. A total of 14 BSG SNe metallicity in the production of exploding BSGs has not been are known in the literature, including SN 1987A. We gathered possible until recently, because SN 1987A belonged to a class of photometric and spectroscopic data for 12 of them (the metal- its own for many years, although a handful of poorly observed licity of SN 1987A is known and we did not observe the host SNe were suggested to be similar to it already in the months of SN 2004ek, which was very recently identified as a BSG SN following the explosion of SN 1987A (see e.g. van den Bergh by Arcavi et al. 2012). We rejected SN 1998bt from the sample 1988). Among these SNe, following Pastorello et al. (2012), after measuring its redshift, which made this SN too bright to be we consider only SNe 1909A and 1982F as reliable 1987A-like a SN 1987A-like event (see Sect. 2.2). Therefore, a total of 11 transients. Therefore, it was impossible to say whether the ex- BSG SN host galaxies were directly investigated to obtain their plosion of compact BSGs usually occurs at low metallicity or metallicities. not. Recently, however, two new SNe with BSG progenitors were 2.1. Photometry presented by Kleiser et al.